Abstract The i.r. spectra of 3,3-dimethyldiaziridine and 3,3-dimethyl- d 6-diaziridine have been recorded for the gaseous and crystalline states between 300 and 4000 cm −1. An absorption spectrum of solid 3,3-dimethyldiaziridine between 70 and 300 cm −1 was also measured. Raman spectra of liquid and crystalline 3,3-dimethyldiaziridine have been recorded from 100 to 4000 cm −1. A vibrational assignment is developed for the molecule in terms of the C 2 point group and the proposed description for the 33 normal modes is based upon isotopic shift ratios, frequency perturbations resulting from hydrogen bonding, and by comparing the data with assignments presented previously for other three-membered ring compounds.